1. Field of the Invention
The present invention relates to a dielectric filter, and more particularly, to a dielectric filter suitable for use as a band-elimination filter in a mobile communication device or the like.
2. Description of the Related Art
FIG. 36 illustrates a conventional band-elimination filter including a dielectric resonator 121, a coupling capacitor 122, and a lead terminal 123 connecting the coupling capacitor 122 to the dielectric resonator 121.
The dielectric resonator 121 is composed of a rectangular dielectric block 124 having a through-hole 125. The inner wall of the through-hole 125 is covered with an inner conductor 126. The outer surface of the dielectric block 124 is covered with an outer conductor 127. One end of the inner conductor 126 is connected to the outer conductor 127. The coupling capacitor 122 is composed of a dielectric substrate 128 having capacitor electrodes 129 and 130 formed on either side of the dielectric substrate 128.
The inner conductor of the dielectric resonator 121 is connected to one capacitor electrode 129 of the coupling capacitor 122 via the lead terminal 123. The other capacitor electrode 130 of the coupling capacitor 122 is connected to a signal line disposed on a circuit board. The outer conductor 127 is connected to a ground line disposed on the circuit board. The dielectric filter having the above structure acts as a band-elimination filter with an equivalent circuit shown in FIG. 37.
As described above, the conventional dielectric filter includes not only the dielectric resonator 121 but also the coupling capacitor 122 and the lead terminal 123. As a result, troublesome manipulation is required to mount a dielectric filter of this type on a circuit board.
FIG. 38 illustrates a typical frequency characteristic obtained in a conventional dielectric filter of the type described above. As can be seen from FIG. 38, the dielectric filter has a simple trap frequency ft with no attenuation in frequency bands around the trap frequency ft. Therefore, when it is desirable that the filter have attenuation property in a frequency band either higher or lower than the trap frequency, it is required to couple the filter with another dielectric filter acting as a band-pass filter. This makes it more difficult to mount the filters.